The field of hydraulics can generally be categorized as high pressure hydraulics, i.e. hydraulics in which the pressures involved are over 5,000 psi, and low pressure hydraulics in which the pressures are less than 5,000 psi. Low pressure hydraulic applications include such things as automotive hydraulics, vehicular leveling jacks and slide-out systems. High pressure hydraulics, on the other hand, are in a different class and generally regarded as an “industrial only” application due to the special expertise that is required when dealing with these applications.
In high pressure hydraulics, the forces generated by the high pressures, which can be in excess of 10,000 psi, can make controlling the forces difficult. Further exacerbating the problem is that in some applications, for example, holding the load with a cylinder, no or only minimal leakage is permissible. Therefore, valves must not only be operable at the high pressure, but also must not leak in these applications.
In addition, some applications require position or speed adjustment, in addition to on and off operation. For these applications, a typical solution is to provide one valve to act as an on/off valve, and another valve, like a needle valve, to perform a metering function. This requires operating two valves in concert with one another to turn on the flow and adjust it.
In addition, some of these same applications can require a more rapid return function, for example if the load is to be returned quickly to a starting or at rest position. Thus, in these applications, a valve with a rapid return function is needed.